It has been proposed to use GNSS-devices (GNSS=global navigation satellite system, such as GPS) on board of vehicles and other objects, such as cranes, to generate proximity warnings in order to reduce the risk of collisions. Such a system is e.g. described in WO 2004/047047. The system is based on apparatus mounted to the objects. Each apparatus comprises a GNSS receiver, a radio transceiver for wireless exchange of the positional data with the other apparatus, and a display device for outputting proximity warnings.
Typically, this type of apparatus is fixedly mounted to vehicles.